全文获取类型
收费全文 | 53930篇 |
免费 | 4839篇 |
国内免费 | 8792篇 |
专业分类
化学 | 49514篇 |
晶体学 | 1674篇 |
力学 | 914篇 |
综合类 | 335篇 |
数学 | 2817篇 |
物理学 | 12307篇 |
出版年
2024年 | 96篇 |
2023年 | 537篇 |
2022年 | 1288篇 |
2021年 | 1231篇 |
2020年 | 1473篇 |
2019年 | 2371篇 |
2018年 | 1424篇 |
2017年 | 2460篇 |
2016年 | 1947篇 |
2015年 | 1790篇 |
2014年 | 2396篇 |
2013年 | 4559篇 |
2012年 | 3207篇 |
2011年 | 3682篇 |
2010年 | 2897篇 |
2009年 | 3328篇 |
2008年 | 3580篇 |
2007年 | 3612篇 |
2006年 | 3380篇 |
2005年 | 3083篇 |
2004年 | 2891篇 |
2003年 | 2366篇 |
2002年 | 2001篇 |
2001年 | 1701篇 |
2000年 | 1647篇 |
1999年 | 1280篇 |
1998年 | 1126篇 |
1997年 | 988篇 |
1996年 | 817篇 |
1995年 | 818篇 |
1994年 | 663篇 |
1993年 | 580篇 |
1992年 | 491篇 |
1991年 | 342篇 |
1990年 | 213篇 |
1989年 | 200篇 |
1988年 | 174篇 |
1987年 | 106篇 |
1986年 | 101篇 |
1985年 | 103篇 |
1984年 | 81篇 |
1983年 | 46篇 |
1982年 | 75篇 |
1981年 | 81篇 |
1980年 | 63篇 |
1979年 | 80篇 |
1978年 | 49篇 |
1977年 | 40篇 |
1976年 | 29篇 |
1973年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Akiko Saito Mana Emoto Akira Tanaka Yuki Doi Kazuaki Shoji Yoshiyuki Mizushina Hiroshi Ikawa Hiromi Yoshida Nobuyasu Matsuura Noriyuki Nakajima 《Tetrahedron》2004,60(52):12043-12049
A simple method for the synthesis of procyanidin B3 substituted with a galloyl group at the 3 and 3″ position is described. Condensation of a benzylated catechin-3-O-gallate electrophile with a nucleophile, catechin and catechin-3-O-gallate, proceeded smoothly and stereoselectively to afford the corresponding dimer gallates, procyanidin B3-3-O-gallate and procyanidin B3-3,3″-di-O-gallate, in good yields. Further, their antioxidant activities on UV-induced lipid peroxide formation, DPPH radical scavenging activity and inhibitory activity of DNA polymerase were also investigated. Among three procyanidin B3 congeners (procyanidin B3, 3-O-gallate and 3,3″-di-O-gallate), the 3,3″-di-O-gallate derivative showed the strongest antioxidant and radical scavenging activity. Interestingly, the 3-O-gallate derivative was the strongest inhibitor of mammalian DNA polymerase with IC50 value of 0.26 μM, although it showed the weakest antioxidant and radical scavenging activity. It became apparent that the presence of a galloyl group at the C-3 position in the proanthocyanidin oligomer was very important for biological activity, however, the antioxidant activity of these compounds was not parallel to the DNA polymerase inhibitory activity. 相似文献
962.
微波溶样肉桂基荧光酮和表面活性剂连续荧光测定葡萄酒中铁和钴 总被引:2,自引:0,他引:2
观察到肉桂基荧光酮(略作CF)在pH≈8与Fc(Ⅲ)配位后荧光增强,CPB的加入使荧光更强,络合物与试剂荧光强度之比F_(FeR)/F_R随0~0.01μg/ml Fe(Ⅲ)线性变化,λ_(ex)=410nm,λ_(ex)=480nm;而CF在pH≈10与Co(Ⅲ)配位后荧光却减弱,乳化剂OP存在时使试剂荧光增强,若以荧光熄灭Co(Ⅲ)灵敏度便提高,试剂与络合物荧光强度之比F_R/F_(CoR)对0~0.016 μg/ml Co(Ⅲ)呈直线关系,λ_(ex)=300nm,L_(em)=352nm。控制化学条件和激发、发射波长,可不经分离测定葡萄酒(微波溶样)中铁和钴。 相似文献
963.
964.
Four new stationary phases for HPLC were prepared by modifying silica gel with a trifrnctional aminoalkyl silane. A conjugated π-electron system was linked to the amino group either directly or with a carboxylic anhydride as a spacer in between. Characterization and dynamic measurements of the new stationary phases were performed by solid state NMR spectroscopy. The results of the 29si and 13C CP/MAS techniques were compared with the recently developed 1H MAS-only technique. Despite strong homonuclear dipole-dipole interactions it was possible to obtain well resolved 1H MAS spectra of those stationaru phases with a high degree of crosslinking. Limited mobility of the aromatic ligand fragments is common for all examined stationaryu phases. The chromatographic properties of the new phases were tested by their ability tio sepatate a mixture of eight PAHS. It could be shown that the π-π interaction mechanism is responsible for the separation of the eight PAHs, because the elution order of the PAHs did not changem despite the use of both a nonpolar mobile phase (n-heptane) and a polar mobile phase (methanol/ water mixture). 相似文献
965.
Cerium phosphate nanoparticles with diameters of 10-180 nm were synthesized by a variety of solution techniques. X-ray diffraction (XRD) determined the crystalline phase(s) present in each sample. Population, shift, and spin-lattice relaxation 31P solid-state nuclear magnetic resonance (NMR) measurements accounted for all the 31P nuclei expected in each sample, and were able to distinguish between phosphorous nuclei in different environments and phases. Transmission electron microscopy (TEM) characterized the morphology and crystallinity of the powder samples as well as of the sintered compacts of the powders. In conjunction with TEM, energy-dispersive spectroscopy (EDS) provided a measure of the composition of the bulk intergranular regions within each CePO4 sample. The presence of an amorphous, phosphate-rich intergranular phase was found in those samples prepared by dissolution of ceria in H3PO4 under various conditions. 相似文献
966.
Polycrystal Li2B4O7 (LBO) doped with Cu and In was prepared and then sintered at different temperatures. X-ray diffraction (XRD) was applied
to get the parameters of the LBO structure, thermoluminescence (TL) and optically stimulated luminescence (OSL) were measured,
and a second-order exponential decay model was fitted to the OSL decay curves. The results indicate that the original number
of OSL traps that have captured electroncs is linearly related with the sum of TL decay during the OSL process. Mean decay
constant of OSL is related to the sintered temperature. The possible reason is that the sintered temperature affects the crystal
sizes of the polycrystal, and consequently affects the stimulating lights’s intensity and the photoionization cross-section
of the electrons, which have been captured by the traps.
__________
Translated from Acta Scientiarum Naturalium Universitatis Sunyatseni, 2005, 44(4) (in Chinese) 相似文献
967.
Victor A.?ZammitEmail author Nigel T.?Price Vicky N.?Jackson Byun-Sung?Park 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1299-1309
Summary. Carnitine acyltransferases catalyse equilibria between acyl-CoA esters and the respective acylcarnitines. Therefore, they act not only as pathway enzymes, but also as modulators of acyl-CoA concentrations within individual sub-cellular compartments. Because acyl-CoA esters are potent biologically active metabolites, carnitine acyltransferase activities are potentially able to affect a diverse range of physiological processes, ranging from insulin secretion, to appetite control, and insulin sensitivity of tissues. The distinctive subcellular distributions of the different types of carnitine acyltransferases also enables them to participate in the transfer of acyl moieties across intracellular membranes, and of particular acylcarnitine esters across the plasma membrane and into the plasma. Pharmacological strategies that make use of these properties to improve cell function are discussed. 相似文献
968.
Stortz CA 《Journal of computational chemistry》2005,26(5):471-483
The 1992 version of MM3 was largely used for modeling mono-, di-, and trisaccharides. In later versions of MM3 improvements were made in some parameters that may be important for carbohydrates. This corrected MM3 force field is part of the Tinker package, freely available (as its 4.1 version), and included in the Chem 3D Ultra 8.0 package (as the 3.7 version). The latter version lacks the corrections to the standard bond lengths produced by electronegativity and anomeric effects, whereas the Tinker 4.1 version only lacks the latter correction. The present work compares the performance of the three MM3 versions (and in some cases, DFT and/or HF/ab initio procedures) on several carbohydrate model problems as the chair and rotamer equilibria in 2-hydroxy- and 2-methoxytetrahydropyran, hydrogen bonding in cis-2,3-dihydroxytetrahydropyran, and the potential energy surfaces around the glycosidic bonds of two sulfated disaccharides and two trisaccharides. Tinker MM3 can be used accurately to estimate carbohydrate energies and geometries, and-with the help of some programming-to pursue studies on the potential energy surfaces of di- and trisaccharides. In most cases results obtained using the three MM3 versions are similar, although large energy differences are obtained when comparing a rotameric distribution around a O-C-O-H dihedral, which is almost forced to the exo-anomeric position by the Tinker versions. In other systems smaller energy differences are found, but they can nevertheless lead to a different global minimum when comparing conformers of similar energy. MM3(92) establishes better the differences between the bond lengths in both anomers, as an expected expression of the anomeric correction. 相似文献
969.
Runru Liu Dapeng Xu Shuang Li Zhe Lü Yanfeng Xue Deyong Wang Wenhui Su 《Frontiers of Chemistry in China》2006,1(4):398-401
In this paper, perovskite oxide SmCoO3 was prepared by the solid-state reaction method using Co2O3 and Sm2O3 as raw materials. The structure and properties of the samples were investigated by XRD, Raman spectral techniques, and DC
measurements and so on. The results of XRD and Raman spectra showed that the mixtures of Co2O3 and Sm2O3 can react to produce a single phase perovskite oxide SmCoO3 around 1353 K. The single-phase SmCoO3 changes from an insulator to a semi-conductor and transition occurs around 470 K. The thermal expansion coefficient (2.17
× 10−5 K−1) of the single-phase SmCoO3 is approximately equal to that of doped LaGaO3, but much bigger than that of SDC(Ce0.85Sm0.15O2) above 873 K. 相似文献
970.
热解-还原法制备单分散Fe3O4亚微空心球 总被引:3,自引:0,他引:3
在用模板法水解FeCl3制备单分散聚(苯乙烯-共-丙烯酸)/Fe2O3[P(St-co-AA)/Fe2O3]核壳粒子的基础上, 于N2环境下热解内核直接得到了单分散的磁性Fe3O4亚微空心球. 用透射电镜(TEM)、场发射扫描电镜(FESEM)、X射线衍射(XRD)、振动样品磁强计(VSM)表征并测试了空心微球的结构形貌、成分以及静磁性能. 结果表明, P(St-co-AA)/Fe2O3核壳粒子在热处理过程中, 由于内核热解生成的有机小分子将Fe2O3 壳层同时还原为Fe3O4, 从而生成了粒径和壁厚均匀的单分散Fe3O4亚微空心球. 该空心微球在室温下的饱和磁化强度、剩余磁化强度和矫顽力分别为50.91 A·m2·kg-1、3.97 A·m2·kg-1和2.33 kA·m-1. 相似文献